The present application relates to a wireless communication device that receives and processes an Ultra Wide Band (UWB) signal in which information is loaded on an extremely weak impulse train by use of an ultra-wide frequency band, and particularly to a wireless communication device that measures the distance between objects by utilizing UWB communication with a transmitter based on a weak impulse train.
More specifically, the present application relates to a wireless communication device that determines the position of a transmitter by utilizing information on the timing of reception of a UWB signal from the transmitter, and particularly to a wireless communication device that executes digital processing for a received UWB signal to thereby detect the reception timing.
Recent trends of wireless LAN systems toward higher speed and lower costs are significantly increasing the demands therefor. In particular, the introduction of Personal Area Networks (PAN) is currently being studied in order to establish a small-scale wireless network among plural electronic apparatuses existing around a person and thus implement information communication in the network. For example, by utilizing frequency bands that need no license from regulatory authorities, such as the 2.4 GHz band and 5 GHz band, different wireless communication systems and wireless communication devices are provided.
Furthermore, in recent years, a wireless communication system called Ultra Wide Band (UWB) communication in which information is loaded on an extremely weak impulse train to implement wireless communication is attracting attention as a wireless communication system that can realize ultra-high speed transmission in short distances. The practical use thereof is expected.
In the United States, the Federal Communication Commission (FCC) has eased regulations regarding the UWB systems in February, 2002 (refer to e.g. Shigenobu Sasaki, Tetsushi Ikegami, and Yukitoshi Sanada, “UWB shisutemu to gijutsu ni kansuru kokusaikaigi (UWBST2002) houkoku (Reports on international conference (UWBST2002) regarding UWB systems and techniques)” (SST2002-19, July, 2002)). The easing of regulations is to allow emission of radio waves with output power up to −41.3 dBm/MHz in the frequency range from 3.1 GHz to 10.6 GHz. Currently, in IEEE 802.15.3 and so forth, it has been proposed to use a transmission method for data with a packet structure including a preamble, as an access control method in Ultra Wide Band communication (refer to e.g. http://grouper.ieee.org/groups/802/15/pub/SG4a.html).
The UWB system has a high time resolution because of the employment of ultra short pulses, and thus can implement ranging, i.e., radar operation and positioning, with use of this characteristic. In particular, recent UWB communication can achieve a function of high-speed data transmission over 100 Mbps in addition to the original ranging function (refer to e.g. JP-A-2002-517001).
It is expected that, in the future, Wireless Personal Access Networks (WPAN) of short-distance communication typified by UWB are included in various appliances and Consumer Electronics (CE) apparatuses. Therefore, besides high-speed data transmission, another added value of wireless communication is expected. Specifically, utilization of position information due to ranging, such as navigation and Near Field Communication (NFC), is expected. Accordingly, it will be desirable to provide a wireless communication device with a ranging function in addition to a high-speed data transmission function.
FIG. 14 schematically illustrates the configuration of a ranging system that employs UWB communication. The illustrated system is based on the premise that UWB communication is utilized not for data transmission but mainly for ranging. A transmitter is formed of a small device such as an IC chip or IC tag, and transmits a UWB signal including a known pattern. Receivers existing around the transmitter detect information on the reception timing of a received UWB signal to thereby detect the distance to the transmitter and the position of the transmitter.
In order for the receiver to detect the distance and position, it is needed for the receiver to acquire precise information on the reception timing of a UWB signal.
For example, a technique has been proposed in which a UWB signal is subjected to analog processing to thereby detect reception timing information (refer to e.g. Y. Shimizu and Y. Sanada, “Accuracy of Relative Distance Measurement with Ultra Wideband System” IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences, vol. J86-A, no. 12, pp. 1310-1319, December 2003 (in Japan)). The circuit configuration of this technique is based on an analog circuit, shown in FIG. 15, that includes an antenna part, a template generation circuit, a loop filter and a clock generation circuit. The correlation values between a signal received by the antenna part and a signal generated by the template generation circuit are calculated. The loop filter detects the reception timing based on the correlation values, and corrects the reception timing for the clock generator.
However, in order to process a UWB signal, which is a high frequency signal, with analog components, a large number of high-accuracy analog components are required, which causes an increase in costs of the receiver. In addition, an increase in implementation errors is expected, and therefore there is difficulty in design of a high frequency circuit.
When the reception timing of a wideband signal such as a UWB signal is detected with use of an analog circuit, complexity of the circuit cannot be avoided due to the configurations of a delay circuit and a frequency synthesis circuit. Accordingly, use of a method in which digital processing is executed for a received UWB signal to thereby detect the reception timing is conceived. In the digital processing, however, the setting of the sample rate and resolution (the number of bits) of an analog-to-digital (A/D) converter is an issue.
For example, a proposal has been made on a UWB receiver that under-samples a high frequency UWB signal (refer to e.g. Hyung-Jin Lee, Dong Sam Ha, and Hyng-Soo Lee, “Toward Digital UWB Radios: Part I-Frequency Domain UWB Receiver with 1 bit ADCs” (Joint UWBST & IWUWBS 2004, in Kyoto)). In this UWB receiver, an A/D converter with a resolution of one bit is used, and a large number of analog filter banks are used to obtain correlations on the frequency axis. Therefore, the configuration of the analog circuit is complicated. In addition, the UWB receiver itself does not have a ranging function.
As yet another technique, a system has been proposed in which digital processing is implemented after the frequency of a received signal is down-converted, to thereby detect reception timing information (refer to e.g. R. D. Gaudenzi, M. Luise, and R. Viola, “A digital chip timing recovery loop for band limited direct-sequence spread-spectrum signals” (IEEE Trans Commun., vol COM-41, pp. 1760-1769, November 1993)). As shown in FIG. 16, the system includes an antenna part, an A/D converter and a timing detection circuit for digital processing for a CDMA signal in a narrow band. A signal received by the antenna part is converted into a baseband signal, followed by being converted into a digital signal by the A/D converter at the chip rate. The timing detection circuit then detects the reception timing.
Such a system in related art that detects reception timing with a digital circuit is premised on low-speed A/D conversion and digital processing at the chip rate. Therefore, the timing detection circuit has a configuration like one shown in FIG. 17. However, since a UWB signal has a symbol length shorter than that of a narrow band CDMA signal, high-speed A/D conversion and high-speed digital processing are needed. Therefore, execution of complicated digital processing is difficult, and is expected to increase the power consumption and circuit area.
It is not easy to realize, with the circuit configuration of FIG. 16, a system that executes digital processing after down-conversion of the frequency of a received signal. A UWB signal does not employ a carrier, and therefore is difficult to convert into a baseband signal. Consequently, processing is implemented for a signal of which frequency is still high, which requires a high-speed A/D converter and high-speed digital processing. Execution of complicated digital processing is difficult, and is expected to increase the power consumption and circuit area. If a down-conversion circuit based on an analog circuit is added, costs of the receiver increase.
FIG. 18 shows another example regarding a system in related art that executes digital processing for a received signal to detect the reception timing. This system includes an antenna part, an A/D converter, an averaging filter, and a timing detection circuit. A signal received by the antenna part is converted into a digital signal by the A/D converter, followed by being subjected to pre-processing in the averaging filter. The timing detection circuit then detects the reception timing. The resolution and speed of the A/D converter is determined based on the noise level and band of the received signal to be processed by the A/D converter. In general, a method is known in which a high-resolution and high-speed A/D converter is used to detect timing (refer to e.g. Y. Shimizu and Y. Sanada, “Relative Distance Measurement with Ultra Wideband System with High Speed 1.5 bit A/D Converter” (Proc. of the Seventh International Symposium on Wireless Personal Multimedia Communications, vol. 1, pp. 50-54, Abano Terme, Italy, September, 2004)).
The system shown in FIG. 18 includes an A/D converter with a resolution equal to or larger than 1.5 bits. In this case, input to the averaging filter are samples having positive and negative values. FIG. 19 illustrates a configuration example of an used averaging filter in related art. In order to implement averaging for sample values that are output from an A/D converter and have positive and negative values, Arithmetic Logical Units (ALU) that can add positive and negative values and a register for temporarily storing operation results are required. In addition, there is a need to drive N ALUs in parallel (N denotes the number of samples in one frame time period) in order to implement averaging for the samples in parallel on each received frame unit basis. Therefore, if the sample rate is high, circuit design is difficult and power consumption is large.
Specifically, it is not easy to construct a system that executes digital processing for a received UWB signal and then detects the reception timing, with use of a high-resolution and high-speed A/D converter like one shown in FIG. 18 (adoption of a high-resolution A/D converter in an actual portable apparatus is impractical even if it can be used in an experimental test machine). When a high-resolution and high-speed A/D converter is used, a high-speed averaging filter becomes necessary, which causes an increase in costs of the receiver. In addition, the costs and power consumption of the A/D converter itself also problematically increase.
Another proposal has been made on a UWB device that can suppress power consumption and can shorten a signal capturing time period not only in communication but also in positioning and ranging (refer to e.g. Japanese Patent Laid-open No. 2005-51466). This UWB device enlarges the width of a pulse included in a UWB signal by letting the UWB signal pass through a low-pass filter, to thereby enable conversion of the UWB signal into a digital signal by use of an A/D converter of a low sample rate. However, if the waveform of a received signal is disturbed due to a multipath below the sample rate of the A/D conversion, the UWB device is problematically affected by the waveform disturbance. In addition, in A/D conversion of a long pulse, a received signal is input to plural correlators and peak search is carried out based on the outputs from the respective correlators to thereby change the sampling timing. However, the execution of the peak search involves problems that the UWB device is susceptible to noises and the positioning distance is transparently decreased.
It is desirable to provide an excellent wireless communication device that can adequately measure the distance between objects by utilizing UWB communication with a transmitter based on a weak impulse train in particular.
It is further desirable to provide an excellent wireless communication device that executes digital processing for a received UWB signal and thus can detect the reception timing.
It is still further desirable to provide an excellent wireless communication device that executes digital processing for a received UWB signal with use of an A/D converter having an adequate sample rate and resolution, and thus can obtain precise reception timing information.